A model for the interaction of near-infrared laser pulses with metal powders in selective laser sintering

A thermal model of the interaction of pulsed near-infrared laser radiation from a Nd:YAG laser was made, taking the measured powder properties such as reflectance, optical penetration depth and thermal conductivity into account. It allows an estimation of the evolution of two different temperatures: the average temperature of the powder (taken over the grains in a volume given by the laser beam diameter and the optical penetration depth) and the temperature distinction within a single grain. It showed that in pulsed mode consolidation can be achieved at much lower average power as the surface of the powder particles are molten but their cores remain at nearly room temperature. This leads to a much lower average temperature and therefore a dramatic decrease in residual thermal stresses in the finished piece. The results of the model were experimentally tested and confirmed. Received: 26 July 2001 / Accepted: 23 November 2001 / Published online: 23 January 2002     

Design, realisation and analysis of an apodised, film-loaded acousto-optic tunable filter

The design and realisation of a SiO₂ film-loaded Ti:LiNbO₃ integrated acousto-optic tunable filter are presented. An optimised Hamming apodisation is achieved through introducing an angular offset between the acoustic and optical waveguides along with carefully placed acoustic absorbers. The realised device has a 20-mm acoustic interaction length and achieves a 3-dB linewidth of 1.8 nm with 19-dB sidelobe suppression. The realised device is highly efficient, requiring a RF drive power of only 10 mW to achieve complete polarisation conversion. The enhanced drive efficiency of the device is analysed using both acoustic and optical mode analysis. Based on this analysis, possible explanations for the improved behaviour are presented and directions that may lead to further enhancements are discussed. Received: 18 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

Optical characterization of thin thermal oxide films on copper by ellipsometry

A complete optical characterization in the visible region of thin copper oxide films has been performed by ellipsometry. Copper oxide films of various thicknesses were grown on thick copper films by low temperature thermal oxidation at 125 °C in air for different time intervals. The thickness and optical constants of the copper oxide films were determined in the visible region by ellipsometric measurements. It was found that a linear time law is valid for the oxide growth in air at 125 °C. The spectral behaviour of the optical constants and the value of the band gap in the oxide films determined by ellipsometry in this study are in agreement with the behaviour of those of Cu₂O, which have been obtained elsewhere through reflectance and transmittance methods. The band gap of copper oxide, determined from the spectral behaviour of the absorption coefficient was about 2 eV, which is the generally accepted value for Cu₂O. It was therefore concluded that the oxide composition of the surface film grown on copper is in the form of Cu₂O (cuprous oxide). It was also shown that the reflectance spectra of the copper oxide–copper structures exhibit behaviour expected from a single layer antireflection coating of Cu₂O on Cu. Received: 19 July 2001 / Accepted: 27 July 2001 / Published online: 17 October 2001     

Broadening of resonances in yttrium nanoparticle optical spectra

The dielectric function of yttrium in the range between 0.2 μm and 2 μm is composed of a harmonic oscillator contribution due to a discrete interband transition and the contribution of free electrons. Hence, it is possible to discuss surface plasmon polaritons as well as other electronic resonances in the optical extinction spectra of yttrium nanoparticles. For the latter, we discuss the broadening of the resonance caused by the aggregation of particles. When particles are lumped into aggregates, the color of the particle system also changes. Aggregation also affects the surface plasmon resonance in yttrium nanoparticles in a way comparable to silver or gold nanoparticle aggregates. Comparison is made with the first experimental results on yttriumnanoparticles, showing that aggregation is the dominant effect for the broad resonance in the measured extinction spectra. Received: 2 July 2001 / Revised version: 10 September 2001 / Published online: 15 October 2001     

All-optical power limiting in waveguides with periodically distributed Kerr-like non-linearity

A slab waveguide with an embedded non-linear grating is considered as an all-optical power-limiting device. The limiting action is based on the power losses at boundaries between grating sections, where a mode-mismatch results from the self-focusing of the fundamental mode in each non-linear section. Time-harmonic and optical pulse propagation are both modelled numerically by finite-difference techniques to demonstrate the behaviour of the device. Received: 17 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

An opto-mechanical nanoshell–polymer composite

Metal nanoshells, which are nanoparticles consisting of a dielectric core surrounded by a metal shell, have an optical response dictated by the plasmon resonance. This optical resonance leads to large extinction cross-sections, which are typically several times the physical cross-section of the particles. The wavelength at which the resonance occurs depends on the core and shell sizes, allowing nanoshells to be tailored for applications. In this paper, we demonstrate how incorporating nanoshells transforms a thermoresponsivepolymer into a photothermally responsive nanoshell–polymer composite. When the thermoresponsive polymer, co-N-isopropylacrylamide-acrylamide (NIPAAm-co-AAm), is heated, the polymer undergoes a reversible decrease in volume. Pristine NIPAAm-co-AAm does not inherently absorb visible or near infrared light. However, by incorporating metal nanoshell particles with a resonance that has been placed at 832 nm into the NIPAAm-co-Aam, nanoshell–polymer composite hydrogels are fabricated. When the composite is illuminated with a diode laser at 832 nm, the nanoshells absorb light and convert it to heat. This induces a reversible and repeatable light-driven collapse of the composite with a weight change of 90% after illumination at 1.8 Wcm^-2. Received: 18 July 2001 / Published online: 10 October 2001     

The photorefractive effect at large modulation depth in semiconductors with multiple defect levels

The photorefractive effect in semiconducting materials with multiple defects is studied in the case of modulation depth m=1. The basic equations are Poisson’s equation and the continuity equations for electrons, holes and occupied defect levels. They include all recombination and optical generation mechanisms between the defect levels and valence and conduction bands. Their explicit numerical solution yields microscopic quantities such as space- and time-dependent electrical field profiles, carrier concentrations, as well as generation and recombination rates. The fundamental Fourier component of the electric field yields the two-wave-mixing gain. Application is made for InP with two levels in the forbidden gap, for which steady-state and transient resulting quantities are shown. The resulting features at large modulation depth are of non-sinusoidal shape. Due to the complexity of the system, the final results strongly depend on all parameters intervening in the models used, as is illustrated for several typical cases. Received: 14 August 2001 / Revised version: 16 October 2001 / Published online: 29 November 2001     

Electromagnetic interaction of fluorophores with designed two-dimensional silver nanoparticle arrays

We study the fluorescence enhancement of dye molecules adsorbed on regular two-dimensional arrays of designed silver nanoparticles. The silver particles show two orthogonal optical resonances at different wavelengths because of their elongated shape. The short-wavelength resonance was designed to fit to the absorption maximum of the fluorophore. When the excitation light drives the short-wavelength resonance, the measured fluorescence intensity is strongly enhanced compared to that for the orthogonal particle orientation. This shows directly a strong electromagnetic coupling between the nanoparticles and the fluorophore. Additionally enhanced photochemical bleaching is observed due to the interaction of fluorophores with the particles. Using a rate model describing the fluorescence enhancement and the bleaching enhancement, an average value for the particle-induced increase in the radiative fluorescence rate is obtained, together with a lower limit for the averaged particle-induced field intensity enhancement factor. Received: 3 July 2001 / Revised version: 3 September 2001 / Published online: 15 October 2001     

Experimental determination of the multiple-scattering effect on the lidar-signal polarization characteristics during liquid- and solid-phase precipitation

We present results of experimental investigations of the signal-polarization characteristics in the case of lidar sounding during precipitation. We show and discuss the lidar signals and the depolarization profiles along the sounding path for liquid- and solid-phase precipitation. In the former case we compare the signal characteristics at different degrees of precipitation rate. In the latter situation, we consider snowfall with particle shape close to that of Chebyshev particles. We also follow the lidar-signal changes depending on the field-of-view of the receiving optics. The experimental data are compared with results of theoretical estimates and models concerning the optical and microphysical characteristics of the rain and snow particles. In the case of liquid-phase precipitation – rain – the observed dependence of the lidar’s signal-polarization structure on the precipitation intensity has two aspects: on the one hand, the change of the raindrops’ shape, and, on the other, the multiple-scattering effects. The lidar data demonstrate that the signal depolarization, and, more specifically, its behavior along the sounding path, can be used as a criterion for the presence of multiple scattering. In the case of a snowfall consisting of Chebyshev particles, the simultaneous role is evident of two factors influencing the lidar-signal depolarization, namely, the non-spherical shape of the particles and the multiple-scattering effects. When the scattering takes place off particles with a large size and a shape strongly differing from spherical, we observed the predominant role of the non-sphericity of the scattering centers in the signal depolarization. Received: 6 December 2000 / Revised version: 11 July 2001 / Published online: 19 September 2001     

Ion synthesis and laser annealing of Cu nanoparticles in Al2O3

Al₂O₃ samples with Cu nanoparticles, synthesised by ion implantation at 40 keV with a dose of 1×10¹⁷ ion/cm² and a current density from 2.5 to 12.5 μA/cm², were annealed using ten pulses from a KrF excimer laser with a single pulse fluence of 0.3 J/cm². The copper depth distribution, formation and modification of metal nanoparticles under the ion implantation and laser treatment were studied by Rutherford backscattering (RBS), energy dispersive X-ray (EDX) analysis, atomic force microscopy (AFM) and optical spectroscopy. It was found that laser annealing leads to a reduction in the nanoparticle size without diffusion of metal atoms into the bulk. The change in particle size and the possibility for oxidation of the copper particles are examined in the framework of Mie theory. Calculations presented show that under excimer laser treatment, Cu nanoparticles are more likely to be reduced in size than to undergo oxidation. Received: 19 April 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

Production of “dichroitic” diffraction gratings in glasses containing silver nanoparticles via particle deformation with ultrashort laser pulses

By repetitive irradiation of pico- or femtosecond laser pulses on glasses containing silver nanoparticles, dichroitic areas can be produced with different optical properties depending on the actual irradiation parameters. This effect, which is nanoscopically caused by permanent deformation of the initially spherical particles to non-spherical shapes and an additional formation of a halo of very small particles, is studied as a function of polarization and number of the applied pulses using two different laser systems (Ti:sapphire, λ=400 nm, t_p=150 fs; Nd:YLF, λ=523.5 nm, t_p=4 ps). A very special diffraction grating produced by this local deformation, which has strongly polarization- and wavelength-dependent features, is introduced and discussed. Received: 20 July 2001 / Published online: 10 October 2001     

Atom force microscopy study on HTHP as-grown diamond single crystals

For understanding the mechanism of diamond growth at high temperature–high pressure (HTHP) from a metallic catalyst–graphite system, it is of great interest to perform atomic force microscopy (AFM) experiments, which provide a unique technique different from that of normal optical and electronic microscopy studies, to study the topography of HTHP as-grown diamond single crystals. In the present paper, we report first AFM results on diamond single crystals grown from a Fe-Ni-C system at HTHP to reveal the growth mechanism of diamond single crystals at HTHP. AFM images for as-grown diamond samples show dark etch pits on the (111) surface, indicating dislocations. Some fine particles about 100–300 nm in dimension were directly observed on the (100) diamond surface. These particles are believed to have been formed through transition of graphite to diamond under the effect of the catalyst and to have been transported to the growing diamond surface through a metallic thin film by diffusion. The roughness of the (100) diamond surface is found to be about several tens of nanometers through profile analysis. The diamond growth at HTHP, in a sense, could be considered as a process of unification of these fine diamond particles or of carbon-atom-cluster recombination on the growing diamond crystal surface. Successive growth interlayer steps on the (111) diamond surface were systemically examined. The heights of the growth interlayer steps were measured by sectional analysis. It was shown that the heights of the growth interlayer steps are quite different and range from about 10 to 25 nm. The source of the interlayer steps might be dislocations. The diamond-growth mechanism at HTHP could be indicated by the AFM topography of the fine diamond particles and the train-growth interlayer steps on the as-grown diamond surfaces. Received: 29 March 2001 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Controlled two-pulse generation in Q-switch mode from a single laser using Q-modulator with frustrated total internal reflection

In this work a cavity design for double-pulse generation in Q-switched mode from a single laser is proposed, based on the construction of a second laser channel using a FTIR (frustrated total internal reflection) Q-modulator. A time interval between the two pulses from 500 ns to 120 μs is obtained in a Nd:YAG laser. A comparison with other methods and cavity designs for double-pulse generation is presented. The case when this technique is applied on a tunable laser with metastable upper laser level (Cr:LiCAF, Cr:LiSAF, Alexandrite, Co:MgF₂ or other) is also considered. Even though the method presented in the paper does not rely only on the cavity configuration proposed, some advantages can be obtained – both polarization and wavelength-independent tuning without polarization and wavelength restrictions in combination with the possibility of different wavelengths and polarizations in each pulse. Moreover, by using an active medium generating wavelengths around and up to 3 μm, the Pockels-cell-Q-switch optical transmission problems can be avoided. Received: 9 May 2001 / Revised version: 2 August 2001 / Published online: 15 October 2001     

An in-depth analysis of reflections in MMI couplers using optical low-coherence reflectometry: design optimization and performance evaluation

This paper describes a comprehensive report on the high potential of optical low-coherence reflectometry (OLCR) measurements to assist the design optimization and performance evaluation of symmetric MMI couplers. Using three sets of deep-ridge InGaAsP–InP couplers on InP and by performing OLCR measurements simultaneously in reflection and transmission modes, the nature and origin of all reflections in MMI have first been identified with respect to a deliberately chosen design parameter, namely, the coupler length. In addition, and in total agreement with the self-imaging principle and also the BPM simulations, the back reflections in combiner mode are shown to be prominent even in fully optimized devices. Finally, a simple design artifact has been proposed and demonstrated experimentally for the first time to suppress back reflections. Received: 16 May 2001 / Published online: 2 November 2001     

Abnormal reflecting mirror structures for intra-cavity Čerenkov second-harmonic generation

Abnormal reflecting mirror (ARM) structures, consisting of a corrugated optical waveguiding structure, can serve as a wavelength-selective end mirror in a laser cavity. The ARM structure shows, for each wavelength in a certain region, 100% reflection at a certain angle of incidence. In the vicinity of this angle the waveguide is resonantly excited, leading to strong enhancement of the optical field in the layer structure, which is interesting for efficient second-harmonic generation (SHG). In this paper, experimental results of a first prototype, exhibiting Čerenkov SHG, are reported. Received: 16 May 2001 / Revised version: 28 August 2001 / Published online: 30 October 2001     

Threshold reduction of stimulated Brillouin scattering (SBS) using fiber loop schemes

SBS mirrors as self-pumped and easy to handle non-linear optical devices are frequently used in high-power laser systems for improving the beam quality based on optical phase conjugation. Because of the non-linear behaviour, a certain pulse energy or power of incident light is needed to generate enough reflectivity for practical purposes. Therefore, reducing this “threshold” is still a main topic in the development of new schemes for optical phase conjugation. In addition to the taper concept reported earlier, this paper deals with loop schemes for reducing the power requirements. A reduction of the so-called “threshold” by a factor of between two and four was obtained with the schemes investigated using liquids and fibers. Received: 4 September 2001 / Revised version: 22 October 2001 / Published online: 23 November 2001     

Pulsed solid-state laser system with fiber phase conjugation and 315 W average output power

Solid-state laser sources with high average output power and high beam quality are required for numerous applications in industry and science. To improve the beam quality, stimulated Brillouin scattering (SBS) mirrors based on optical phase conjugation are used to compensate for phase distortions in master oscillator power amplifier (MOPA) systems. SBS phase conjugation in commercially available silica optical fibers facilitates the construction of high-power solid-state laser systems with high beam quality. Here we present a flash lamp-pumped, passively Q-switched Nd:YAP system which delivers an average output power of 315 W with M²=2.6 at a wavelength of 1.08 μm. Received: 11 July 2001 / Revised version: 28 September 2001 / Published online: 23 November 2001     

Metallic waveguide mirrors in polymer film waveguides

A technology for the fabrication of metallic waveguide mirrors is developed. Plane and curved waveguide mirrors, the latter acting in the same way as cylindrical lenses, are realized in benzocyclobutene (BCB) film waveguides. The waveguide mirror structure is dry-etched into the BCB film waveguide. To enhance the reflectivity of the waveguide mirrors, the waveguide edge is metallized. The BCB film waveguide mirrors are characterized with respect to waveguide attenuation and mirror reflectivity. The waveguide attenuation of the processed BCB waveguide is 0.5 dB/cm. Ag-coated BCB waveguide mirrors show a reflectivity of 71%. The efficiency of total internal reflection (TIR, i.e. in the case without metallization) at the dry-etched waveguide edge is 74%. As an application of the BCB waveguide mirrors a hybrid integrated optical module for Fourier-optical transverse mode selection in broad area lasers (BAL) is proposed. Received: 16 May 2001 / Published online: 23 October 2001     

The color of finely dispersed nanoparticles

We discuss the dependence of the color of low-concentrated nanoparticle systems on particle size and mass concentration for Ag, Au and TiN nanoparticles, which exhibit a surface plasmon polariton resonance in extinction spectra. Comparison is made with color data obtained for Ag and Au colloidal suspensions. When particles lump into aggregates, the splitting of the surface plasmon resonance into new resonances affects the extinction of light and, hence, the color of the particle assembly. This is demonstrated for aggregated colloidal suspensions of Ag and Au nanoparticles. Finally, for highly concentrated assemblies such as pigment films, we discuss the dependence of the color in diffuse reflectance and transmittance according to Kubelka and Munk (P. Kubelka, F. Munk: Z. Techn. Phys. 12, 593 (1931)), and extend this model by using optical properties of aggregates of spheres. Received: 2 July 2001 / Published online: 10 October 2001     

Composite spatial solitons in a saturable nonlinear bulk medium

We review the generation of the recently predicted multi-component spatial optical solitons in a saturable nonlinear bulk medium. We present numerical simulations for an effectively isotropic model and experimental results for a set of different combinations of a Gaussian beam co-propagating incoherently with a beam of a more complex internal structure, such as a higher order transverse laser mode. We discuss the different formation processes and the general properties of a variety of different dipole-mode composite solitons and expand our investigations to the generation of a quadrupole-mode composite soliton. Received: 1 December 2000 / Revised version: 12 January 2001 / Published online: 21 March 2001